1. Field of the Invention
The present invention relates to a color cathode ray tube and more specifically to a color cathode ray tube in which beam landing errors caused by non-uniform thermal expansion of a shadow mask are corrected such that color purity is improved.
2. Description of the Background Art
FIG. 1 shows a schematic diagram illustrating the structure of a general color cathode ray tube of the background art. As shown in FIG. 1, the color cathode ray tube generally includes a glass envelope having a shape of bulb and is comprised of a faceplate panel 10, a tubular neck 120, and a funnel 20 connecting the panel 10 and the neck 120.
The panel 10 comprises a faceplate portion and a peripheral sidewall portion sealed to the funnel 20. A phosphor screen 30 is formed on the inner surface of the faceplate portion. The phosphor screen 30 is coated by phosphor materials of R, G, and B. A multi-apertured color selection electrode, i.e., shadow mask 40 is mounted to the screen with a predetermined space. The shadow mask 40 is supported by a peripheral frame 70. An electron gun 50 is mounted within the neck to generate and direct electron beams 60 along paths through the mask to the screen.
The shadow mask 40 and the frame 70 constitute a mask-frame assembly. The mask-frame assembly is joined to the panel 10 by means of springs 80.
The cathode ray tube further comprises an inner shield 90 for shielding the tube from external geomagnetism, a reinforcing band 100 attached to the sidewall portion of the panel 10 to prevent the cathode ray tube from being exploded by external shock, and external deflection yoke 110 located in the vicinity of the funnel-to-neck junction.
The electron beams generated by the electron gun are deflected in either vertical or horizontal directions by the deflection yoke 110. The electron beams are selected by the shadow mask depending on the colors and impinge on the phosphor screen such that the phosphor screen emits light in different colors. Typically, about 80% of the electrons from the electron gun 50 fail to pass through the apertures of the shadow mask 40. The 80% of electrons impinge upon the shadow mask 40, producing heat and raising the temperature of the mask 40.
FIG. 2 shows a perspective view of a lower right quarter of a shadow mask illustrating thermal distribution of the surface of the mask due to the impingement of electrons. As shown in FIG. 2, the temperature of the mask is different for different portions of the mask. In FIG. 2, a center portion of the mask has a higher temperature than a corner portion. The reason why the corner portion has a lower temperature is that the heat at the corner portion is dissipated through the frame attached to the mask. Since the frame is attached to the mask at the skirt portion near the corner, heat at the corner is easily transferred to the outside via the frame. Because the mask is thermally expanded, a position of the apertures at the shadow mask is shifted from the desired position accordingly. Therefore, electron beams passing through the apertures land at the screen incorrectly. In this way the color purity at the screen is degraded. This phenomenon of purity degradation resulting from the undesired positional shift of the apertures of the mask is called the “doming effect.”
FIG. 3a shows a cross-sectional view of the shadow mask for illustrating purity degradation resulting from the positional shift of the apertures of the shadow mask 40. FIG. 3b is a graph showing the extent of variation in the positional shift of electrons landing incorrectly at the screen with respect to time when the cathode ray tube is placed in operation.
As shown in FIG. 3a, an electron beam landing at the screen is shifted due to the positional shift of the apertures of the shadow mask. As shown in FIG. 3b, the extent of the shift of the electron beam landing at the screen increases just after the cathode ray tube is operated, since the temperature of the shadow mask begins to increase. However, as the heat at the shadow mask is transferred to the frame, the frame is heated and expanded. Accordingly, the positional shift of the electron landing is decreased. As the heat dissipation through the frame continues, the landing position of the electron beam is displaced in the opposite direction with respect to the initial shift, which occurs just after the initial operation of the shadow mask.
The variation in the shift of the electron beam landing causes degradation of color purity. Further, since the landing position varies in accordance with the time after the shadow mask is operated, restoration of the aperture position with respect to the screen is difficult.
FIG. 4 is a perspective view of the conventional shadow mask. The conventional shadow mask comprises a central apertured portion 41 through which electron beams pass, a non-apertured border portion 42 surrounding the apertured portion 41, and a peripheral skirt portion 43 bent back from the border portion 42 and extending backward from the apertured portion 41. As shown in FIG. 4, the border portion 42 and the skirt portion 43 have more area than is necessary in view of the function they perform. The large area of the border portion 42 and the skirt portion 43 increases the non-uniformity of thermal expansion across the shadow mask. Therefore, the conventional shadow mask suffers from color purity degradation caused by the doming effect.
Moreover, the welding point between the shadow mask and the frame intensifies the non-uniformity of the thermal expansion. Typically, the shadow mask is fixed to the frame by welding through a plurality of welding points 43a. When the shadow mask expands thermally due to the beam radiation, the welding points become binding points against the expansion of the shadow mask. Therefore, the non-uniformity of expansion of the shadow mask is increased, thereby increasing a landing error of the electron beams.
In order to prevent or lessen the doming effect caused by a landing error of the electron beams, many different approaches have been used.
First, structural improvements of the shadow mask have been suggested in order to prevent the landing error problem. According to Japanese Laid-Open Patent Publication No. S62-177831, a temperature control device is provided within the cathode ray tube in order to suppress the temperature elevation of the mask. Also, according to Japanese Laid-Open Patent Publication No. H6-267446, a reinforcement member for maintaining the shape of the shadow mask is provided between the shadow mask and the frame. However, the landing error problem was not solved by those structural approaches.
Also, improvement in the material used for the shadow mask was suggested. Invar material having a low thermal expansion rate was used for the shadow mask instead of aluminum killed (AK) material. However, the result of using the invar material was not satisfactory in view of the price of the material.
Finally, there have been many approaches to solve landing errors caused by spring back phenomenon. Spring back phenomenon occurs when the shadow mask is manufactured by a forming process. When a forming process is used in making a shadow mask, a shadow mask is formed by pressing to have a shape comprising a central portion and a skirt portion bent back from the central portion 41 and extending backward. Then, the shadow mask is fixed to a frame. After the mask-frame assembly is made, the skirt portion of the shadow mask tends to move outward from the center by a resilient force. This is called spring back phenomenon. This spring back phenomenon is one of the causes of the landing error problem.
As a solution for solving the landing error problem due to the spring back phenomenon, an idea of making the border portion of the shadow mask to be partially thinner than the central portion was suggested in Japanese Laid-Open Patent Publication No. S49-112566. Additionally, according to Japanese Laid-Open Patent Publication No. S63-271849, protrusions are provided, which are protruded from a skirt portion of a shadow mask backward from a central portion. According to Japanese Laid-Open Patent Publication No. H1-169847, many openings are perforated in the skirt portion for absorbing compression stress. However, those techniques are directed to solving the landing error problem caused by the spring back phenomenon. Therefore, those techniques are not sufficient to solve the problem due to the non-uniform thermal expansion of the shadow mask.